1. Technical Field of the Invention
The invention relates to polarization-independent integrated optical devices and, more particularly, to polarization-independent optical devices employing substrates having coefficients of thermal expansion approximating that of subsequently-deposited doped glass layer(s).
2. Background
As optical fiber communication channels replace metal cable and microwave transmission links, integrated optical devices for directly processing optical signals become increasingly important. A useful approach to optical signal processing employs integrated glass waveguide structures formed on silicon substrates. The basic structure of such devices is described in C. H. Henry et al., "Glass Waveguides on Silicon for Hybrid Optical Packaging," 7 J. Lightwave Tech., pp. 1530-1539 (1989), the disclosure of which is incorporated by reference herein. Integrated glass waveguide structures are typically formed by depositing a base layer of silicon dioxide on a silicon substrate followed by deposition of a doped silica layer on the silicon dioxide to form a waveguide core. The doped silica core layer is patterned using standard lithographic techniques to form a desired waveguide structure. For single mode waveguide structures, the core layer is approximately 6-8 microns thick and 5-7 microns wide. Following patterning of the core layer, an additional layer of silica is deposited to act as a top cladding. Depending upon the precise configuration of the waveguide, such devices can perform a variety of functions such as beam-splitting, tapping, multiplexing, demultiplexing, and filtering.
One drawback of the above waveguide structures is strain-induced birefrigence in the waveguide core. Compressive strain is introduced during fabrication due to different thermal expansion between silicon and silica. Due to this birefrigence, different polarization modes of transmitted light are presented with different effective indices of refraction. Specifically, the transverse magnetic (TM) mode encounters a greater refractive index than does the transverse electric (TE) mode, adversely affecting the transmission of the light and the performance of the waveguide circuit. This effect in which the two components of the polarization travel at different velocities is further aggravated by curves in the waveguide, since optical modes are shifted radially outward when traversing a curve. A mode loosely bound to the waveguide core (TM) will experience a greater outward shift than a mode more tightly bound (TM). Consequently, the loosely bound mode has a greater optical path and phase.
Elimination of birefrigence has long been recognized as necessary for high-performance optical devices. The art is replete with procedures for reducing or compensating for the compressive strain induced by the silicon/silica thermal mismatch. One approach involves deposition of a thick amorphous silicon layer on the waveguide followed by trimming with a high power laser. See M. Kawachi et al., "Laser Trimming Adjustment of Waveguide Birefrigence in Silica Integrated Optic Ring Resonators," Proc. CLEO '89, Tu J. (17) (Apr., 1989). In another approach, deep grooves on the order of 60 microns are etched adjacent the waveguide to release strain. Both these techniques require extreme precision, and significantly increase manufacturing costs. Additionally, these techniques attempt to correct the strain induced by processing without addressing the fundamental problem--thermal mismatch among the materials of the waveguide structure.
There is a need in the art for integrated optical waveguide structures and methods for their manufacture having minimal strain-induced birefrigence. Such structures could be used to produce polarization-independent integrated optical devices.